Various configurations are available for improving the frictional force on ice of a pneumatic tire (studless tire) for ice-bound or snow-covered roads. Here, there is one configuration in which numerous cells are formed in a tread rubber. In this configuration, the absorbing and removing of a water film on an ice surface when the tread comes in contact with the ice surface; and releasing of the water by centrifugal force when the tread is separated from the ice surface are repeated to improve the frictional force on ice. Japanese patent Publication No. H10-316801 proposes, as a method for forming such cells, that a rubber composition for a tire tread contains heat-expandable microcapsules, and the microcapsules are expanded by heat during the vulcanization step to form resin-encapsulated cells.
Additionally, in a studless tire, the hardness of the tread rubber is kept low even at low temperature so that an adhesion property to an ice surface is enhanced, thereby improving the frictional force on ice. Here, the rubber flexibility can be secured at low temperature when the tread rubber contains silica as a method for improving the frictional force on ice. Nevertheless, the studless tire, in which micro-sized resin-encapsulated cells are formed when the tread rubber contains heat-expandable microcapsules as described above, has the following problem. In a case where the tread rubber contains silica, shell materials of the respective microcapsules are broken by silica while the rubber composition is being kneaded, and thereby the microcapsules cannot expand during vulcanization molding. For this reason, desired resin-encapsulated cells cannot be formed, and thereby satisfactory frictional force on ice cannot be obtained.
Japanese Patent Application Publication Ne. 2003-105138 proposes the following method as a countermeasure against this problem. Specifically, the kneading of the rubber composition is achieved through the first and second operations. First, rubber and silica are kneaded as the first operation. Then, microcapsules are mixed into the mixture of rubber and silica as the second operation. Nevertheless, even this method cannot fully prevent the microcapsules from breaking during the mixing of the microcapsules. For this reason, the method needs to be further improved.